EFFECTS OF POWER-PLANT OPERATION ON ZOOPLANKTON 



631 



20 



10 

 



Asplanchna spp. 



I • I  I  I  I  I 



E 



6 



> 



LU 

 Q 



80 

 60 

 40 

 20 



30 



20 



10 







Year 4 



1 I I 



Year 5 



Ploesoma truncatum 



Year 6 



T^ } I 



JASONDJ FMAMJ JASONDJ FMAMJ JASCNDJ FMAMJ J 

 1973 1 1974 ) 1975 1 1976 



Fig. 7 Seasonal distribution of five rotifer species from Belews 

 Lake. Data plotted from mean monthly euphotic-zone density, all 

 stations. 



noted when year 6 was compared to year 5. When all stations were 

 pooled and years compared, no significant differences occurred. This 

 was due to high year— month interaction. Variation between years 

 was nonparallel. 



Monthly sampling of Polyarthra spp. yielded a seasonal pattern 

 with one to two peaks. Peaks were compared with average surface 

 temperatures and two optimums were found, one at 25 to 27°C and 

 one at 12 to 16°C. This suggests that at least two species were 

 important. 



